Spark plug



Oct.17, 1939. E. F. KOEHLE R El AL SPARK PLUG Filed Jan. 6, 1937 v I by insulation designated generally at l6.

Patented Oct. 17, 1939 SPARK PLUG Ernest F. Koehler, South Berlin, and Richard Harden, Boston, Mass., assignors, by mesne assignments, to Ernest F. Koehler, South Berlin, Mass, and Gustave W. Goerner, Bourne, Mass.

Application January 6, 1937, Serial No. 119,290

2 Claims.

and methods of making the same.

The object of the present invention is to provide a spark plug which aifords exceptionally high insulation between the electrodes, which is unbreakable, which maintains the proper sparking gap throughout its life and which is substantially free from carbonization.

With this object in view the principal feature m of the invention comprises a spark plug having inner and outer electrodes which have flush or nearly flush surfaces at the sparking zone. The electrodes are separated by annular insulation which comprises wound sheet material of small thickness, this material being preferably sheet mica which is excellent for both its electrical and heat resisting characteristics.

Other features 01 the invention consist of certain novel features of construction and modes of 20 operation hereinafter described and particularly defined in the claims.

In the accompanying drawing, Fig. 1 is a side elevation of one form of spark plug according to the present invention; Figs. 2 and 3 are deii tail views illustrating steps in the assembly; Fig. 4 is a bottom plan view of the spark plug; Fig. 5 is a'detail sectional view illustrating another modified form of the invention; and Fig. 6

is a sectional elevation of a modified form of 80. spark plug according to the present invention.

The spark plug illustrated in Figs. 1 to 4 in- 'clusive comprises an outer electrode orshell 8 having the usual threads H) for reception in a cylinder head, and an inner concentric elec- 86 trode l2. The shell 8 has a taper bore and the inner electrode I2 is in the form of a pin having a corresponding taper. The pin is provided at its upper end with a threaded portion H.

The inner and outer electrodes are separated This ,insulation comprises thin sheet material, preferably mica, wound up to produce the required insulation thickness. For example, with mica sheets two-thousandthsof an inch thick, ten 4| convolutions are required to produce a twentythousandths gap. Owing to the taper of the inner electrode the mica sheet from which the insulation is wound is necessarily curved as indicated at IS in Fig. 2. The mica is wound on 50 a mandrel to the proper size and thickness. The tightly wound mica roll is then secured by a strip 01' thin paper adhesive tape 20. The mica roll is inserted into the taper bore of the shell. An inner electrode l2'is then inserted v 55 into the mica roll as shown in Fig. 3 and the (Cl. 123-169) The present invention relates to spark plugs inner electrode and the roll are forced into the shell by a suitable press. In this operation the mica is tightly compressed by the pressure be-- tween the tapering walls of the electrodes. When the inner electrode is forced into its prop- I er position the parts are rigidly held against displacement. 'As shown by a comparison of Figs. 1 and 3, the mica roll and the taper pin are originally of somewhat greater length than in the final assembly, so that after the pressing 10 operation they will protrude beyond the bottom end of the shell. The inner electrode and the I mica are later to be machined ofi flush with the outer electrode as shown in Fig. 1.

After the pressing operation, the plug is provided with external body insulation, indicated in Fig. 1 as a series of mica washers 22 having an internal diameter sumcient to surround the mica roll IS. A retaining cap 24 is then threaded on the upper end of the electrode pin with suflicient force to compress the washers 22.

The insulation 22 is machined to provide a smooth exterior appearance and the protruding ends of the inner electrode I2 and the mica roll l6 are then machined off so that at the bottom 25 face the surface of both electrodes and the mica are accurately flush. A terminal cap 26 of any suitable construction is then threaded on the upper end of the pin.

The construction described above is advan- 3o tageous in that the bottom surfaces of both electrodes and the mica are substantially flush so that no indentation or protrusion of the mica with respect to the metal surfaces can be detected. In normal operation, the spark takes place between the inner or outer electrodes over the entire annulus or at more or less separated points. The flush machining of the mica with respect to the metal protects the sharp edges of the metal and assures against any change in 40 the gap which might arise due to pitting of the metal. Furthermore, since the insulation is. directly at the zone of sparking and is thereby raised to approximately the same temperature as the adjacent metal parts; carbonization of the mica is practically avoided.

. The modified form of the invention shown in Fig. 5 is similar to that previously described except that the inner electrode is provided at the.

bottom with a protruding portion 28 of increased taper. In this construction the spark takes place between the edge of the portion 28 and the shell. This form of the invention does not provide for sparking over flush surfaces, but it has been found that carbonization and wear are very small in comparison with spark plugs of the usual'type. Furthermore, the construction has the manufacturing advantage that it permits the use of mica rolls of standard form, the gap being varied-by using pins with different protruding in fact the gap may be changed even after'ajssembly by milling 08 some of the protruding po ion 28 either at the bottom or around the periphery.

The construction shown in Fig. 6 embodies a cylindrical inner electrode and hence ofi'ers the advantage of lovver manufactm-ing cost. The

shell is bored with cylindrical walls, the bore haviiig a shoulder 32. Received in the bore is a composite mica roll comprising a roll a of small external diameter andprotruding above the shell, and a second or bottom roll 36 of larger diameter, the upper part of which bears against the shoulder 32. Preferably the rolls are wound with an overlap, the first few convolutions of the roll 36 being wound into the last few convolutions of the roll 34. The composite roll as thus made up is inserted into the shell and the bottom surface is finished oil flush with the shell. The inner electrode pin is then inserted. The pin is provided at the bottom with a conical head 40 provided with internal prongs 42 which are forced into the roll 38. The shoulder 32 positions both the mica roll and the inner electrode while the prongs I! prevent rotation under subsequent application of the retaining cap 24. The mica washers 22 and the cap 24 are applied in the manner previously described. Although this construction employs more mica, it has the manufacturing advantage over the taper construction that the mica is not required to be cut having a taper bore, an inner electrode having a taper corresponding to the taper bore, the inner electrode protruding upwardly above the shell, a winding of thin sheet insulation tightly compressed annuiarly between the inner electrode and the shell and surroundingthe inner electrode above the shell, and a body of exterior insulation above the shell and surrounding the wound insulation, the inner electrode having a portion of increased taper protruding slightly beyond and flaring outwardly under the wound insulation at the bottom. v I

2. A spark plug comprising a shell electrode having at the top a cylindrical bore and at the bottom a bore of larger'diameter, a composite winding of thin sheet insulating material fitting closely within both bores and protruding upwardly above the top of the shell and substantially flush with the bottom of the shell, and an inner electrode fitting within the winding and having a head abutting the bottom surface thereof, the head having projections engaging with the winding.

ERNEST F. KOEHLER.

, RICHARD HARDENQ 

